[petsc-users] SLEPc eigensolver that uses minimal memory and finds ALL eigenvalues of a real symmetric sparse matrix in reasonable time

[Jose E. Roman]

El 09/08/2011, a las 09:54, Shitij Bhargava escribió:

> Thanks Jose, Barry.
> 
> I tried what you said, but that gives me an error:
> 
> [0]PETSC ERROR: --------------------- Error Message ------------------------------------
> [0]PETSC ERROR: Argument out of range!
> [0]PETSC ERROR: Can only get local values, trying 9!
> 
>  This is probably because here I am trying to insert all rows of the matrix through process 0, but process 0 doesnt own all the rows.
> 
> In any case, this seems very "unnatural", so I am using MPIAIJ the right way as you said, where I assemble the MPIAIJ matrix in parallel instead of only on one process. I have done that actually, and am running the code on the cluster right now. Its going to take a long long time to finish, so I cant confirm some of my doubts, which I am asking below:
> 
> 1. If I run the code with 1 process, and say it takes M memory (peak) while solving for eigenvalues, then when I run it with N processes, each will take nearly M/N memory (peak) (probably a little more) right ? And for doing this, I dont have to use any special MPI stuff....the fact that I am using MPIAIJ, and building the EPS object from it, and then calling EPSSolve() is enough ? I mean EPSSolve() is internally in some way distributing memory and computation effort automatically when I use MPIAIJ, and run the code with many processes, right ?
> This confusion is there because when I use top, while running the code with 8 processes, each of them showed me nearly 250 mb initially, but each has grown to use 270 mb in about 70 minutes. I understand that the method krylovschur is such that memory requirements increase slowly, but the peak on any process will be less (than if I ran only one process), right ?  (Even though their memory requirements are growing, they will grow to some M/N only, right ?)

The solver allocates some dynamic memory when the actual computation starts, so it is normal that you see a growth in the memory footprint. No further increase should be observed afterwards.

Jose 

> 
> Actually the fact that in this case, each of the process creates its own EPS context, initializes it itself, and then calls EPSSolve() itself without any "interaction" with other processes makes me wonder if they really are working together, or just individually (I would have verified this myself, but the program will take way too much time, and I know I would have to kill it sooner or later).....or the fact that they initialize their own EPS context with THEIR part of the MPI is enough to make them "cooperate and work together" ? (Although I think this is what Barry meant in that last post, but I am not too sure)
> 
> I am not too comfortable with the MPI way of thinking right now, probably this is why I have this confusion.
> 
> Anyways, I cant thank you guys enough. I would have been scrounging through documentation again and again to no avail if you guys had not helped me the way you did. The responses were always prompt, always to the point (even though my questions were sometimes not, probably because I didnt completely understand the problems I was facing.....but you always knew what I was asking) and very clear. At this moment, I dont know much about PETSc/SLEPc myself, but I will be sure to contribute back to this list when I do. I have nothing but sincere gratitude for you guys.
> 
> 
> Thank you very much !
> 
> Shitij